Method and device for subsea dredging

ABSTRACT

Method and device for moving subsea rocks and sediments, particularly at significant depths, for example in connection with removal of protective rocks around subsea installations where maintenance is to be conducted. The device includes a rigid or at least partly flexible tubing thorough which masses may be transported with the aid of a pressure gradient produced by an ejector nozzle arranged externally in relation to the tubing, and fed with water from a water pump. The device further includes a chassis adapted to be transported along the sea bottom. The required power is supplied through a cable from the surface, while the tubing preferably is remotely controlled by a manipulator.

The present invention relates to a method of the kind described in thepreamble of claim 1. The invention further relates to a device accordingto the preamble of claim 3 for conducting said method.

BACKGROUND OF THE INVENTION

For work at subsea oil and gas installations or in connections with suchinstallations, e.g. maintenance work, there is often a need to moverocks and particulate material that partly covers the bodies that are tobe repaired. It can be pipelines, valve housings and the like.

In a similar way a need may occur to remove sediments in connection withnew installations on the sea bottom, or for removal of collected drillcuttings at platforms or the like.

Similar needs may also occur in connection with subsea work, likeharbour works or work at barrage or quay structures.

DESCRIPTION OF RELATED ART

The most common way to remove sediments in connection with subsea work,is by utilizing large “fans”, large and heavy suction devices with ahigh power consumption and specially designed excavators. Disadvantagesare that they require a lot of power and/or other resources, theyrequire large surface vessels, have a limited versatility, are as goodas stationary, or they are not at all suited for deep waters.

NO patent No. 302.043 describes a dredge designed for subsea operations,especially to remove or move drill cuttings, comprising a motor, a pumpdevice and an ejector, where the motor is designed to run the pump whichin its turn provides a stream of water to the ejector, which ispositioned in a tubing through which the cuttings or the like issupposed to be transported. The apparatus is designed to rest on the seabottom and to receive energy from the surface, while the inlet end ofthe tubing is supposed to be moveable e.g. with the aid of a remotecontrolled mini submarine, a so called ROV.

This apparatus is not suited to move sediments with relatively largerocks, mainly because the pipeline has an effective loss of diameter dueto the ejector's design and position. Further it has a geographicallyvery limited work range as it is designed to rest at the sea bottom,even though the pipeline is designed to be somewhat moveable.

Japanese patent applications Nos. 043 25 799 A and 043 25 800 Adescribes an ejector pump system where the ejector is positioned mainlyoutside the pipeline so that the ejector does not reduce the effectivediameter of the pipeline. From the abstract of these patent applicationsit is not possible to see what kind of utilizations these systems aremeant for. Neither are there any indications of dimensions or powerrequirements for these systems.

SUMMARY OF THE INVENTION

It is an object with the present invention to provide a method fortransportation of rocks and sediments under water, especially at deepwaters.

It is a particular objective to provide a method for transportation ofrocks with a typical maximum diameter of 250-500 mm.

It is a further object to provide an apparatus for performing saidmethod, which apparatus should be versatile in its use, especially inthe way that it should be easy to move around down at the sea bottom.

It is a still further object to provide such an apparatus that is easyto control, and which does not require more energy than what may besupplied from the surface, e.g. through a conventional electric cable.

BRIEF DESCRIPTION OF THE DRAWINGS

Below a more detailed description of a device according to the inventionis given with reference to the accompanying drawings, where:

FIG. 1 is a schematic drawing of a first embodiment of the invention,

FIG. 2 is a schematic drawing of a second embodiment of the invention,

FIG. 3 is a simplified schematic drawing of a third embodiment of theinvention,

FIGS. 4 a-c shows details of a device according to the inventionaccording to any one of the embodiments shown in FIGS. 1-3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a device 1 designed to move on the sea bottom S with theaid of belts 2 powered from the surface through a cable 3. The devicecomprises a tubing 5, preferably with a flexible section 8, said tubinghaving an inlet end 6 and an outlet end 7. To the tubing an ejectornozzle 11 is attached, said nozzle being supplied with water from a pump12 powered by an hydraulic unit 13. All of said equipment are supportedby a chassis F which again is supported by the belts 2. It is preferredthat the tubing 5, when it includes a flexible section 8, furthercomprises a manipulator 9 which is able to move the tubing withincertain degrees of freedom. In FIG. 1 the manipulator 9 consists of amulti-link arm controlled by means of an hydraulic unit 16. The deviceis adapted to transport sediments 14 including rocks of a size up to thediameter of the tubing 5 from one site to another, by the pressuregradient in the tubing set up by the ejector nozzle 11, providing a“vacuum from left to right in the drawing.

FIG. 2 shows an alternative embodiment of the invention. By thisembodiment there is no power to the wheels or belts, as the device issupported by freely moving wheels 2′ capable of being turned in severaldirections and preferably in any direction. The drawing shows 2 wheelswhile it is understood that at least two other wheels are hidden behindthese two. Most typically the device in this embodiment has 4 wheels,but it may also have e.g. 3 or 5 wheels. As an alternative to freelymoving wheels, freely moving belts may be utilized.

By the device according to FIG. 2, the manipulator 9′ consists of aremotely operated vehicle (ROV) controlling the tubing 5 and, if thesea-bottom so allows, the ROV may pull the entire device 1 in a desireddirection. It is to be understood that the freely moving wheels 2′ neednot have the shown shape, they may have any form suited for subseatransportation.

FIG. 3 shows a further embodiment of the device according to theinvention, an embodiment that may be seen as a variation of theembodiment of FIG. 2. FIG. 3 is simplified and does not show all thefeatures of FIG. 2. The central aspect of this embodiment lies in thedetails indicated by the reference numeral 2″, which may be denoted“water cushions” (cf. air cushions of a hovercraft), which may cause thedevice to float just above the sea level. The so-called water cushionsare supplied with water from a powerful water pump, for instance thepump that feeds the ejector nozzle 11. In the drawing this is shownschematically in the form of a particular supply conduit 18 from thepump 12. Movement of tubing 5 and possibly of the entire device 1 may asshown in FIG. 2, be effectuated by means of a pulling force from an ROVthrough the tubing 5.

FIG. 4 shows details at the inlet end 6. The FIG. 4 a shows that theouter part (mouth piece) of the inlet end 6 comprises telescopic units21 may be pulled or pushed out. FIG. 4 b further shows that the mouthpiece may comprise an annulus “lance” 21 which is hollow and which isable to flush water through a plurality of openings 22 inwards as wellas outwards relative to the mouth piece, so that the inlet end as suchbecomes shielded and not so easily will become packed when the mouthpiece is pressed into the sediments. The water is fed to the lancethrough conduit 23 which may communicate with e.g. the water pump 12 oranother suitable water pump.

At the inlet mouth piece 10 of the tubing 5 there may also be provided anozzle (not shown) for backflushing of rocks etc. that might get stuckin the mouth piece.

Further it is preferred that the inlet mouth piece 10 is rounded andthat the cross-section of the tubing is constant, and that any bend onthe tubing 5 has sufficiently large radius to ensure that rocks will notget stuck. It is further preferred that the outlet end 7 of the tubingis shaped as a diffusor, as this reduces the frictional loss through thetubing.

The device according to the invention may be manufactured mainly in aplastic material with a density close to that of water, so that it iseasy to support.

CALCULATION EXAMPLE

In the following example of utilization there is an assumption of one ortwo water pumps each powered by a motor of 75 kW. It is assumed that thetubing has an internal diameter of 300 mm. In the case of two pumpsthere is also conducted calculations for a 500 mm tubing. Further dataare given in the table below.

Motor power (axle-) kW 75 150 150 Power efficiency % 80 80 80 Internaldiameter mm 300 300 500 Length (inlet-outlet) m 15 15 12 Speed prior tomixing m/s 5.8 7.4 5.9 chamber Required speed m/s 4.4 4.4 5.7 Motivepower m 2.5 4.2 1.8 (lifting height) of which inlet loss is m 0.3 0.60.4 frictional loss is m 1.4 2.3 0.7 outlet loss is m 0.7 1.3 0.7 Ca.capacity transport rocks tons/hour 70 120 100

PRACTICAL EXAMPLE

A commission conducted shows that the invention works in practice.During the summer of 1999, 1500 m³ (d_(max)=ca. 150 mm) of rocks weremoved with the aid of a corresponding ejector mechanism, carried by aremotely operated vehicle, ROV. The commission was conducted inTengsfjorden, by an oil pipe at a depth of 540 m below sea level. Forpowering the water pumps, two hydraulic engines with a total effect ofapprox. 24 kW were used. The tubing was 10 m long and had an internaldiameter of 250 mm.

During 26 effective work hours 1500 m³ of rocks were moved, whichcorresponds to a capacity of 60 tons/hour. Only a minimal wearage wasobserved on the tubing in PE plastic. Later, several successful taskshave been performed with this technology.

In March 2000 the present invention was utilized at the Draugen field,at a depth of 300 m. The commission was carried out from the boat SeawayKingfisher. 5 m length of a pipeline was uncovered during 40 minutesbefore the commission had to be interrupted. This corresponds to 20 m³mass or 45 tons/hour. Considering that the rocks were moved from aregion where frequent re-locations of the device was required, theresult was very satisfying. A 75 kW pump and a tubing of 15 m with aninternal diameter of 300 mm was utilized.

The drawings and the examples are merely illustrations of the invention,which is only limited by the subsequent claims.

1. Method for moving subsea rocks and sediments along a sea floor,comprising: arranging on a movable chassis tubing which has asubstantially constant cross-section between a mouth end and an ejectionend, and which is flexible over at least a portion thereof, an ejectornozzle external to the tubing and connected to the tubing between themouth end and the ejection end, a water pump which is connected to theejector nozzle opposite to the connection to the tubing, and a powercable to supply power to the chassis from the sea surface; disposing themovable chassis with tubing, ejector nozzle pump and power cable on thesea floor; producing a pressure gradient in the tubing by pumping waterfrom the water pump through the ejector nozzle, creating thereby suctionat the mouth end of the tubing; and utilizing the suction to move rocksand sediment from a first point on the sea floor adjacent to the mouthend to a second point on the sea floor adjacent to the ejection end. 2.Method as claimed in claim 1, wherein the tubing is remotely controlledby means of a manipulator.
 3. Method as claimed in claim 2, wherein themanipulator is a hydraulically controlled multi-link arm.
 4. Method asclaimed in claim 1, wherein the chassis is provided with belts or wheelsto move the chassis along the sea bottom with power provided to thebelts or wheels.
 5. Method as claimed in claim 2, wherein the chassis isprovided with belts or wheels to move the chassis along the sea bottomwith power provided to the belts or wheels, the belts or wheels areconstructed and arranged to turn freely in several directions, themanipulator is an ROV and the ROV is utilized for moving the chassis aswell as for controlling the tubing.
 6. Method as claimed in claim 5,additionally comprising furnishing the chassis with water cushions thatare fed with water, at least one water pump enabling the chassis tofloat above the sea bottom using an ROV as the manipulator, wherein theROV is utilized for moving the chassis and for controlling the tubing.7. Device for moving subsea rocks and sediments along a sea floor,comprising a chassis constructed and arranged for movement along the seafloor, having disposed thereon: tubing which is flexible over tubingwhich has a substantially constant cross-section between a mouth end andan ejection end, and which is flexible over at least a portion thereof,an ejector nozzle external to the tubing and connected to the tubingbetween the mouth end and the ejector end, a water pump which isconnected to the ejector nozzle opposite to the connection to thetubing, and a power cable to supply power to the chassis from the seasurface, whereby pumping water from the pump through the ejector nozzlecreates suction at the mouth end of the tubing sufficient to pick uprocks and sediment from the sea floor adjacent the mouth end of thetubing, and deposit the rocks and sediment on the sea floor adjacent theejection end of the tubing.
 8. Device as claimed in claim 7,additionally comprising a manipulator to remotely control the tubing. 9.Device as claimed in claim 8, wherein the manipulator is a hydraulicallycontrolled multi-link arm.
 10. Device as claimed in claim 7, wherein thechassis is supported by belts or wheels and is constructed and arrangedto be transported along the sea bottom by power to the belts or thewheels.
 11. Device as claimed in claim 8, wherein the manipulator is anROV that is controlled substantially independently of the device. 12.Device as claimed in claim 11, wherein the chassis is supported byfreely turning belts or wheels that optionally may be turned in anydirection, and is constructed and arranged to be transported along thesea bottom by a pull force exerted by the ROV through the tubing. 13.Device as claimed in claim 11, wherein the chassis is provided withwater cushions facing the sea bottom and fed with water from the waterpump or by and additional pump, whereby the chassis may partly floatabove the sea bottom and is arranged to be transported along the seabottom by a pull force exerted by the ROV through the tubing.
 14. Deviceas claimed in claim 8, wherein the first and second ends of the tubingare disposed adjacent the chassis.